This will be my first reply so please forgive my newness...
I believe that Starkiller was a long term project on the part of Vader, possibly his attempt to create/prolong life.
This thought first occured to me earlier this year when i reread the TFU novel. When Juno is first brought to the secret chamber, it mentions several time that there were cryotubes (or someting similar to that). I wondered why that was so significant, but now it makes sense that deceased 'subjects' get frozen and stored.
If Starkiller got killed, it would make sense that Vader would just unthaw a new one and send him on his next mission.
Slighty more far fetched, I was under the impression that bacta only went so far in healing a paitent. to that end, it was supposed to be a plot device to explain Luke's change of appearance in that it DOESN'T heal every wound. And yet Galen remarked on how his scars were gone and he wasn't robotic.
I propose that there have been more clones, with newer units replacing deceased ones.